Distributed policy-based provisioning and enforcement for quality of service

ABSTRACT

Embodiments of the disclosure provide techniques for measuring congestion and controlling quality of service to a shared resource. A module that interfaces with the shared resource monitors the usage of the shared resource by accessing clients. Upon detecting that the rate of usage of the shared resource has exceeded a maximum rate supported by the shared resource, the module determines and transmits a congestion metric to clients that are currently attempting to access the shared resource. Clients, in turn determine a delay period based on the congestion metric prior to attempting another access of the shared resource.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to,co-pending U.S. patent application Ser. No. 14/010,247, entitled“Distributed Policy-Based Provisioning and Enforcement for Quality ofService,” filed August 26, 2013, granted as U.S. Pat. No. 9,887,924 B2on February 6, 2018, the contents of which are incorporated herein byreference. This application is related to the following commonlyassigned applications: “Load Balancing of Resources” (Ser. No.14/010,275), published as U.S. Publication No. 2015/0058863 A1 onFebruary 26, 2015, “Scalable Distributed Storage Architecture” (Ser. No.14/010,293), granted as U.S Pat. No. 9,811,531 B2 on November 7, 2017,and “Virtual Disk Blueprints for a Virtualized Storage Area Network”(Ser. No. 14/010,316), granted as U.S. Pat. No. 10,747,475 B2 on August18, 2020, each of which was filed on August 26, 2013. Each relatedapplication is incorporated by reference herein in its entirety.

BACKGROUND

Distributed systems allow multiple clients in a network to access a poolof shared resources. For example, a distributed storage system allows acluster of host computers to aggregate local disks (e.g., SSD, PCI-basedflash storage, SATA, or SAS magnetic disks) located in or attached toeach host computer to create a single and shared pool of storage. Thispool of storage (sometimes referred to herein as a “datastore” or“store”) is accessible by all host computers in the cluster and may bepresented as a single namespace of storage entities (such as ahierarchical file system namespace in the case of files, a flatnamespace of unique identifiers in the case of objects, etc.). Storageclients in turn, such as virtual machines spawned on the host computersmay use the datastore, for example, to store virtual disks that areaccessed by the virtual machines during their operation. Because theshared local disks that make up the datastore may have differentperformance characteristics (e.g., capacity, input/output per second orIOPS capabilities, etc.), usage of such shared local disks to storevirtual disks or portions thereof may be distributed among the virtualmachines based on the needs of each given virtual machine.

This approach provides enterprises with cost-effective performance. Forinstance, distributed storage using pooled local disks is inexpensive,highly scalable, and relatively simple to manage. Because suchdistributed storage can use commodity disks in the cluster, enterprisesdo not need to invest in additional storage infrastructure. However, oneissue that arises with this approach relates to contention betweenmultiple clients accessing the shared storage resources. In particular,reduced overall performance and higher latency occur when multipleclients need to simultaneously access different data that is backed bythe same local disk in a particular host computer at a combined IOPS(input/output per second) that exceeds the IOPS capacity of the localdisk.

SUMMARY

One embodiment of the present disclosure provides a method for providingresource usage feedback to a plurality of clients having access to ashared resource. The method generally includes monitoring a rate ofusage of the shared resource by at least a portion of the clients,wherein each client has been reserved a minimum usage rate for theshared resource. Upon detecting that the rate of usage of the sharedresource has exceeded a maximum rate supported by the shared resource,congestion metric is determined for at least a portion of the clientsthat are currently attempting to access the shared resource. Thecongestion metric for each client is based on the client's usage of theshared resource and is used by the clients to calculate a delay periodprior attempting another access of the shared resource. The methodgenerally includes transmitting each of the determined congestionmetrics to a corresponding client.

Other embodiments include, without limitation, a computer-readablemedium that includes instructions that enable a processing unit toimplement one or more aspects of the disclosed methods as well as acomputer system having a processor, memory, and modules configured toimplement one or more aspects of the disclosed methods.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example computing environment, according to oneembodiment.

FIG. 2 illustrates an example hierarchical structure of objectsorganized within an object store that represent a virtual disk,according to one embodiment.

FIG. 3 illustrates components of a VSAN module, according to oneembodiment.

FIG. 4 illustrates a method flow diagram for creating a virtual diskobject based on a defined storage policy, according to one embodiment.

FIG. 5 illustrates the handling of an I/O operation originating from aVM, according to one embodiment.

FIG. 6 illustrates a method flow diagram for transmitting congestionmetrics to clients by a VSAN module, according to an embodiment.

FIG. 7 illustrates a method for delaying an I/O operation request to theVSAN module during congestion.

DETAILED DESCRIPTION

Embodiments disclosed herein provide techniques for measuring congestionat a shared resource (e.g., storage, etc.) and controlling the qualityof service provided to consumers of the shared resource. In oneembodiment, a host computer providing a shared resource (e.g., its localstorage, etc.) to a cluster of host computers monitors the usage of theshared resource (e.g., IOPS, etc.) of clients (e.g., virtual machinesrunning on any of the host computers in the cluster, etc.) accessing theresource. If there is contention between clients for the shared resource(e.g., if certain clients are unable to access the shared resource at aminimum rate that has been reserved to them as further described herein)because certain of such clients are exceeding their own reserved rates,the host computer issues a congestion metric to at least a portion ofthe clients trying to access the shared resource. The congestion metricmay specify the extent of resource contention. Clients use thecongestion metric to determine how long to delay subsequent requests(e.g., I/O operations, etc.) to access the shared resource. For example,in certain embodiments, clients may receive a non-zero congestion metric(e.g., clients that have exceeded their reserved rates) that may causethem to delay requests for a period of time based on the congestionmetric, while other clients may receive a “zero” congestion metric(e.g., clients that have used resources within the rates reserved forthem) may send requests immediately. The computer hosting the storageresources may calibrate the response to the congestion metric so that agiven client has a total service time similar to what the client mayhave had if all requests had been delivered immediately.

For instance, the techniques described herein may be used to implement adistributed storage system where the host computer issues congestionmetrics on I/O operation requests by clients that may be located onother host computers that are accessing the local storage of the hostcomputer. One example of an applicable distributed storage system is asoftware-based “virtual storage area network” (VSAN) where host serversin a cluster each act as a node that contributes its commodity localstorage resources (e.g., hard disk and/or solid state drives, etc.) toprovide an aggregate “object” store. Each host server may include astorage management module (also referred to herein as a VSAN module) inorder to automate storage management workflows (e.g., create objects inthe object store, etc.) and provide access to objects in the objectstore (e.g., handle I/O operations to objects in the object store, etc.)based on predefined storage policies specified for objects in the objectstore. In a particular embodiment, the host servers further support theinstantiation of virtual machines (VMs) which act as clients to the VSANobject store. In such an embodiment, the “objects” stored in the objectstore may include, for example, file system objects that may contain VMconfiguration files and virtual disk descriptor files, virtual diskobjects that are accessed by the VMs during runtime and the like.

An administrator may initially configure a VM with specific storagerequirements for its “virtual disk” depending its intended use (e.g.,capacity, availability, IOPS, etc.), the administrator may define astorage profile or policy for each VM specifying such availability,capacity, IOPS and the like. As further described below, the VSAN modulemay then create a virtual disk for the VM backing it with physicalstorage resources of the object store based on the defined policy. Forexample, an administrator may specify a storage policy for a virtualdisk for VM A that requires a minimum reservation of 400 read IOPS (buthaving no limit on the maximum amount of read IOPS it can consume), andaccordingly, the VSAN module may create a virtual disk “object” backedby a local storage resource that can provide a maximum of 550 read IOPS.Further, the administrator may specify another storage policy for avirtual disk for VM B that requires a minimum reservation of 100 readIOPS (but, again, having no limit on the maximum amount of read IOPS itcan consume). The VSAN module may also back the virtual disk for VM Bwith the same local storage resource (and therefore be left with 50 readIOPS remaining). If VM A exceeds its minimum reservation of 400 readIOPS and, for example, utilizes 500 read IOPS, VM B can continueperforming I/O operations to its virtual disk at a rate of 50 read IOPSwithout experiencing contention for the local storage resource. However,if VM A continues to perform I/O on its virtual disk at 500 read IOPS,and VM B needs to use its minimally reserved amount of 100 read IOPS toaccess its own virtual disk, contention will occur.

In one embodiment, the VSAN module on each node (e.g., host server)monitors the rate at which the local storage resources on the node arebeing accessed by clients (such as, for example, other VSAN modules onother nodes that are acting on behalf of VMs running on such nodes). Inone embodiment, the VSAN module residing in each node may broadcast thecurrent resource usage status to other VSAN modules of other nodes inthe cluster. When contention occurs at a local storage resource, forexample, due to simultaneous access by multiple clients, the nodecalculates a congestion metric that is then issued to the clientsattempting to access the local storage resource. The congestion metricmay indicate a measure of how much contention is occurring, which maycause the clients (e.g., the VSAN modules acting on behalf of VMs thatare performing I/O operations that reach the local storage resource) todelay or otherwise throttle back their I/O operation requests. Incertain embodiments, the VSAN module in the node may also itself use thecongestion metric to queue and delay incoming requests to the localstorage resource.

Alternatively, the VSAN module of the host computer hosting the storageresources calculates the amount of congestion to the resources. Oncedetermined, the VSAN module transmits the congestion metric to theclient without calculating the resource usage of a particular client.Upon receiving the congestion metric from the host computer, the clientcalculates a delay period based on its own resource usage and whether itis exceeding allotted resources.

This approach allows for distributed provisioning and decentralizedpolicy enforcement across multiple clients. By disaggregating a singlelarge queue at a server (e.g., node housing the local storage resource)to a smaller queue at the server and separate queues at each client(where the overall average queuing delay per request is what would beseen using a single large queue), this approach reduces space needed forbuffering at the server while still allowing differential priorityscheduling among the requests of each client. Further, because onlyclients that exceed their reserved allocation for resources (and not theclients that are using resources as reserved for them) delay requests tothe VSAN module, such an approach requires less complicated logic,resulting in improved performance of the distributed system.

Reference is now made in detail to several embodiments, examples ofwhich are illustrated in the accompanying figures. Note, that whereverpracticable, similar or like reference numbers may be used in thefigures and may indicate similar or like functionality. The figuresdepict embodiments for purposes of illustration only. One having skillin the art will readily recognize from the following description thatalternative embodiments of the structures and methods illustrated hereinmay be employed without departing from the principles described herein.

In the following, a VSAN module provides as a reference example of asystem that monitors resources and controls the congestion of resourcesat multiple points. This reference example is included to provide anunderstanding of the embodiments described herein. However, it will beapparent to one of skill in the art that these embodiments areapplicable in other contexts related to allocating distributed storageresources to clients, regardless of the type of computing environment.For example, the embodiments may be applicable to software definedcomputers, networks, and storage arrays. Further, the embodiments mayalso be applicable in other contexts relating to allocating other sharedcomputing resources (e.g., processing, memory and network resources,etc.).

Similarly, numerous specific details are provided to provide a thoroughunderstanding of the embodiments. One of skill in the art will recognizethat the embodiments may be practiced without some of these specificdetails. In other instances, well known process operations andimplementation details have not been described in detail to avoidunnecessary obscuring of novel aspects of the disclosure.

FIG. 1 illustrates a computing environment 100, according to oneembodiment. As shown, computing environment 100 is a VSAN environmentthat leverages the commodity local storage housed in or directlyattached (hereinafter, use of the term “housed” or “housed in” may beused to encompass both housed in or otherwise directly attached) to hostservers or nodes 111 of a cluster 110 to provide an aggregate objectstore 116 to virtual machines (VMs) 112 running on the nodes. The localcommodity storage housed in or otherwise directly attached to the nodes111 may include combinations of solid state drives (SSDs) 117 and/ormagnetic or spinning disks 118. In certain embodiments, SSDs 117 serveas a read cache and/or write buffer in front of magnetic disks 118 toincrease I/O performance.

A virtualization management platform 105 is associated with cluster 110of nodes 111. Virtualization management platform 105 enables anadministrator to manage the configuration and spawning of VMs on thevarious nodes 111. As depicted in the embodiment of FIG. 1, each node111 includes a virtualization layer or hypervisor 113, a VSAN module114, and hardware 119 (which includes the SSDs 117 and magnetic disks118 of a node 111). Through hypervisor 113, a node 111 is able to launchand run multiple VMs 112. Hypervisor 113, in part, manages hardware 119to properly allocate computing resources (e.g., processing power, randomaccess memory, etc.) for each VM 112. Furthermore, as described furtherbelow, each hypervisor 113, through its corresponding VSAN module 114,provides access to storage resources located in hardware 119 (e.g., SSDs117 and magnetic disks 118) for use as storage for virtual disks (orportions thereof) and other related files that may be accessed by any VM112 residing in any of nodes 111 in cluster 110. In a particularembodiment, vSphere Hypervisor from VMware, Inc. (VMware) may beinstalled on nodes 111 as hypervisor 113 and vCenter Server from VMwaremay be used as virtualization management platform 105.

In one embodiment, VSAN module 114 is implemented as a “VSAN” devicedriver within hypervisor 113. In such an embodiment, VSAN module 114provides access to a conceptual “VSAN” 115 through which anadministrator can create a number of top-level “device” or namespaceobjects that are backed by object store 116. In one common scenario,during creation of a device object, the administrator may specify aparticular file system for the device object (such device objectshereinafter also thus referred to “file system objects”). For example,in one embodiment, each hypervisor 113 in each node 111 may, during aboot process, discover a /vsan/ root node for a conceptual globalnamespace that is exposed by VSAN module 114. By, for example, accessingAPIs exposed by VSAN module 114, hypervisor 113 can then determine allthe top-level file system objects (or other types of top-level deviceobjects) currently residing in VSAN 115. When a VM (or other client)attempts to access one of the file system objects, hypervisor 113 maydynamically “auto-mount” the file system object at that time. A filesystem object (e.g., /vsan/fs_name1, etc.) that is accessible throughVSAN 115 may, for example, be implemented to emulate the semantics of aparticular file system such as VMware's distributed or clustered filesystem, VMFS, which is designed to provide concurrency control amongsimultaneously accessing VMs. Because VSAN 115 supports multiple filesystem objects, it is able provide storage resources through objectstore 116 without being confined by limitations of any particularclustered file system. For example, many clustered file systems (e.g.,VMFS, etc.) can only scale to support a certain amount of nodes 111. Byproviding multiple top-level file system object support, VSAN 115overcomes the scalability limitations of such clustered file systems.

As described in further detail in the context of FIG. 2 below, a filesystem object, may, itself, provide access to a number of virtual diskdescriptor files (e.g., .vmdk files in a vSphere environment, etc.)accessible by VMs 112 running in cluster 110. These virtual diskdescriptor files contain references to virtual disk “objects” thatcontain the actual data for the virtual disk and are separately backedby object store 116. A virtual disk object may itself be a hierarchicalor “composite” object that, as described further below, is furthercomposed of “component” objects (again separately backed by object store116) that reflect the storage requirements (e.g., capacity,availability, IOPs, etc.) of a corresponding storage profile or policygenerated by the administrator when initially creating the virtual disk.As further discussed below, each VSAN module 114 (through a clusterlevel object management or “CLOM” sub-module, in embodiments as furtherdescribed below) communicates with other VSAN modules 114 of other nodes111 to create and maintain an in-memory metadata database (e.g.,maintained separately but in synchronized fashion in the memory of eachnode 111) that contains metadata describing the locations,configurations, policies and relationships among the various objectsstored in object store 116. This in-memory metadata database is utilizedby a VSAN module 114 on a node 111, for example, when an administratorfirst creates a virtual disk for a VM as well as when the VM is runningand performing I/O operations (e.g., read or write) on the virtual disk.As further discussed below in the context of FIG. 3, VSAN module 114(through a document object manager or “DOM” sub-module, in oneembodiment as further described below) traverses a hierarchy of objectsusing the metadata in the in-memory database in order to properly routean I/O operation request to the node (or nodes) that houses (house) theactual physical local storage that backs the portion of the virtual diskthat is subject to the I/O operation.

FIG. 2 illustrates an example hierarchical structure of objectsorganized within object store 116 that represent a virtual disk,according to one embodiment. As previously discussed above, a VM 112running on one of nodes 111 may perform I/O operations on a virtual diskthat is stored as a hierarchical or composite object 200 in object store116. Hypervisor 113 provides VM 112 access to the virtual disk byinterfacing with the abstraction of VSAN 115 through VSAN module 114(e.g., by auto-mounting the top-level file system object correspondingto the virtual disk object, as previously discussed, in one embodiment).For example, VSAN module 114, by querying its local copy of thein-memory metadata database, is able to identify a particular filesystem object 205 (e.g., a VMFS file system object in one embodiment,etc.) stored in VSAN 115 that stores a descriptor file 210 for thevirtual disk (e.g., a .vmdk file, etc.). It should be recognized thatthe file system object 205 may store a variety of other files consistentwith its purpose, such as virtual machine configuration files (e.g.,.vmx files in a vSphere environment, etc.) and the like when supportinga virtualization environment. In certain embodiments, each file systemobject may be configured to support only those virtual diskscorresponding to a particular VM (e.g., a “per-VM” file system object).

Descriptor file 210 includes a reference to composite object 200 that isseparately stored in object store 116 and conceptually represents thevirtual disk (and thus may also be sometimes referenced herein as avirtual disk object). Composite object 200 stores metadata describing astorage organization or configuration for the virtual disk (sometimesreferred to herein as a virtual disk “blueprint”) that suits the storagerequirements or service level agreements (SLAs) in a correspondingstorage profile or policy (e.g., capacity, availability, IOPS, etc.)generated by an administrator when creating the virtual disk. Forexample, in the embodiment of FIG. 2, composite object 200 includes avirtual disk blueprint 215 that describes a RAID 1 configuration wheretwo mirrored copies of the virtual disk (e.g., mirrors) are each furtherstriped in a RAID 0 configuration. Composite object 225 may thus containreferences to a number of “leaf” or “component” objects 220 _(x)corresponding to each stripe (e.g., data partition of the virtual disk)in each of the virtual disk mirrors. The metadata accessible by VSANmodule 114 in the in-memory metadata database for each component object220 (e.g., for each stripe) provides a mapping to or otherwiseidentifies a particular node 111 _(x) in cluster 110 that houses thephysical storage resources (e.g., magnetic disks 118, etc.) thatactually store the stripe (as well as the location of the stripe withinsuch physical resource).

FIG. 3 illustrates components of a VSAN module 114, according to oneembodiment. As previously described, in certain embodiments, VSAN module114 may execute as a device driver exposing an abstraction of a VSAN 115to hypervisor 113. Various sub-modules of VSAN module 114 handledifferent responsibilities and may operate within either user space 315or kernel space 320 depending on such responsibilities. As depicted inthe embodiment of FIG. 3, VSAN module 114 includes a cluster levelobject management (CLOM) sub-module 325 that operates in user space 315.CLOM sub-module 325 generates virtual disk blueprints during creation ofa virtual disk by an administrator and ensures that objects created forsuch virtual disk blueprints are configured to meet storage profile orpolicy requirements set by the administrator. In addition to beingaccessed during object creation (e.g., for virtual disks), CLOMsub-module 325 may also be accessed (e.g., to dynamically revise orotherwise update a virtual disk blueprint or the mappings of the virtualdisk blueprint to actual physical storage in object store 116) on achange made by an administrator to the storage profile or policyrelating to an object or when changes to the cluster or workload resultin an object being out of compliance with a current storage profile orpolicy.

In one embodiment, if an administrator creates a storage profile orpolicy for a composite object such as virtual disk object 200, CLOMsub-module 325 applies a variety of heuristics and/or distributedalgorithms to generate virtual disk blueprint 215 that describes aconfiguration in cluster 110 that meets or otherwise suits the storagepolicy (e.g., RAID configuration to achieve desired redundancy throughmirroring and access performance through striping, which nodes' localstorage should store certain portions/partitions/stripes of the virtualdisk to achieve load balancing, etc.). For example, CLOM sub-module 325,in one embodiment, is responsible for generating blueprint 215describing the RAID 1/RAID 0 configuration for virtual disk object 200in FIG. 2 when the virtual disk was first created by the administrator.As previously discussed, a storage policy may specify requirements forcapacity, IOPS, availability, and reliability. Storage policies may alsospecify a workload characterization (e.g., random or sequential access,I/O request size, cache size, expected cache hit ration, etc.).Additionally, the administrator may also specify an affinity to VSANmodule 114 to preferentially use certain nodes 111 (or the local diskshoused therein). For example, when provisioning a new virtual disk for aVM, an administrator may generate a storage policy or profile for thevirtual disk specifying that the virtual disk have a reserve capacity of400 GB, a reservation of 150 read IOPS, a reservation of 300 write IOPS,and a desired availability of 99.99%. Upon receipt of the generatedstorage policy, CLOM sub-module 325 consults the in-memory metadatadatabase maintained by its VSAN module 114 to determine the currentstate of cluster 110 in order generate a virtual disk blueprint for acomposite object (e.g., the virtual disk object) that suits thegenerated storage policy. As further discussed below, CLOM sub-module325 may then communicate the blueprint to its corresponding distributedobject manager (DOM) sub-module 340 which interacts with object store116 to implement the blueprint by, for example, allocating or otherwisemapping component objects (e.g., stripes) of the composite object tophysical storage locations within various nodes 111 of cluster 110.

In addition to CLOM sub-module 325 and DOM sub-module 340, as furtherdepicted in FIG. 3, VSAN module 114 may also include a clustermonitoring, membership, and directory services (CMMDS) sub-module 335that maintains the previously discussed in-memory metadata database toprovide information on the state of cluster 110 to other sub-modules ofVSAN module 114 and also tracks the general “health” of cluster 110 bymonitoring the status, accessibility, and visibility of each node 111 incluster 110. The in-memory metadata database serves as a directoryservice that maintains a physical inventory of the VSAN environment,such as the various nodes 111, the storage resources in the nodes 111(SSD, magnetic disks, etc.) housed therein and thecharacteristics/capabilities thereof, the current state of the nodes 111and their corresponding storage resources, network paths among the nodes111, and the like. As previously discussed, in addition to maintaining aphysical inventory, the in-memory metadata database further provides acatalog of metadata for objects stored in object store 116 (e.g., whatcomposite and component objects exist, what component objects belong towhat composite objects, which nodes serve as “coordinators” or “owners”that control access to which objects, quality of service requirementsfor each object, object configurations, the mapping of objects tophysical storage locations, etc.). As previously discussed, othersub-modules within VSAN module 114 may access CMMDS sub-module 335(represented by the connecting lines in FIG. 3) for updates to learn ofchanges in cluster topology and object configurations. For example, aspreviously discussed, during virtual disk creation, CLOM sub-module 325accesses the in-memory metadata database to generate a virtual diskblueprint, and in order to handle an I/O operation from a running VM112, DOM sub-module 340 accesses the in-memory metadata database todetermine the nodes 111 that store the component objects (e.g., stripes)of a corresponding composite object (e.g., virtual disk object) and thepaths by which those nodes are reachable in order to satisfy the I/Ooperation.

As previously discussed, DOM sub-module 340, during the handling of I/Ooperations as well as during object creation, controls access to andhandles operations on those component objects in object store 116 thatare stored in the local storage of the particular node 111 in which DOMsub-module 340 runs as well as certain other composite objects for whichits node 111 has been currently designated as the “coordinator” or“owner.” For example, when handling an I/O operation from a VM, due tothe hierarchical nature of composite objects in certain embodiments, aDOM sub-module 340 that serves as the coordinator for the targetcomposite object (e.g., the virtual disk object that is subject to theI/O operation) may need to further communicate across the network with adifferent DOM sub-module 340 in a second node 111 (or nodes) that servesas the coordinator for the particular component object (e.g., stripe,etc.) of the virtual disk object that is stored in the local storage ofthe second node 111 and which is the portion of the virtual disk that issubject to the I/O operation. If the VM issuing the I/O operationresides on a node 111 that is also different from the coordinator of thevirtual disk object, the DOM sub-module 340 of the node running the VMwould also have to communicate across the network with the DOMsub-module 340 of the coordinator. In certain embodiments, if the VMissuing the I/O operation resides on node that is different from thecoordinator of the virtual disk object subject to the I/O operation, thetwo DOM sub-modules 340 of the two nodes may communicate so as to changethe role of the coordinator of the virtual disk object to the noderunning the VM (e.g., thereby reducing the amount of networkcommunication needed to coordinate I/O operations between the noderunning the VM and the node serving as the coordinator for the virtualdisk object).

DOM sub-modules 340 also similarly communicate amongst one anotherduring object creation. For example, a virtual disk blueprint generatedby CLOM module 325 during creation of a virtual disk may includeinformation that designates which nodes 111 should serve as thecoordinators for the virtual disk object as well as its correspondingcomponent objects (stripes, etc.). Each of the DOM sub-modules 340 forsuch designated nodes is issued requests (e.g., by the DOM sub-module340 designated as the coordinator for the virtual disk object or by theDOM sub-module 340 of the node generating the virtual disk blueprint,etc. depending on embodiments) to create their respective objects,allocate local storage to such objects (if needed), and advertise theirobjects to their corresponding CMMDS sub-module 335 in order to updatethe in-memory metadata database with metadata regarding the object. Inorder to perform such requests, DOM sub-module 340 interacts with a logstructured object manager (LSOM) sub-module 350 that serves as thecomponent in VSAN module 114 that actually drives communication with thelocal SSDs and magnetic disks of its node 111. In addition to allocatinglocal storage for component objects (as well as to store other metadatasuch a policies and configurations for composite objects for which itsnode serves as coordinator, etc.), LSOM sub-module 350 additionallymonitors the flow of I/O operations to the local storage of its node111, for example, to report whether a storage resource is congested.

FIG. 3 also depicts a reliable datagram transport (RDT) sub-module 345that delivers datagrams of arbitrary size between logical endpoints(e.g., nodes, objects, etc.), where the endpoints may potentially beover multiple paths. In one embodiment, the underlying transport is TCP.Alternatively, other transports such as RDMA may be used. RDT sub-module345 is used, for example, when DOM sub-modules 340 communicate with oneanother, as previously discussed above to create objects or to handleI/O operations. In certain embodiments, RDT module 345 interacts withCMMDS module 335 to resolve the address of logical endpoints dynamicallyin order to maintain up-to-date location information in the in-memorymetadata database as well as to create, remove, or reestablishconnections based on link health status. For example, if CMMDS module335 reports a link as unhealthy, RDT sub-module 345 may drop theconnection in favor of a link in better condition.

FIG. 4 illustrates a method flow diagram for creating a virtual diskobject based on a defined storage policy, according to one embodiment.For example, in step 400, an administrator may interact with a userinterface of virtual management platform 105 to create a virtual diskhaving capacity, availability and IOPS requirements (e.g., the definedstorage policy). In one embodiment, virtual management platform 105 maythen request a “master” node 111 to create an object for the virtualdisk in step 405. In step 410, such a master node 111 may generate avirtual disk blueprint through its CLOM sub-module 325 in VSAN module.As previously discussed, CLOM sub-module 35 generates a virtual diskblueprint for the creation of a virtual disk object (e.g., a compositeobject) based on the status of cluster 110 as determined by consultingthe in-memory metadata database of CMMDS sub-module 335. The virtualdisk blueprint may identify a particular node that should serve as thecoordinator or owner of the virtual disk object. In step 415, the DOMsub-module 340 of the master node 111 may the request the DOM sub-module340 of the identified node to create the virtual disk object. In step420, the DOM sub-module 340 of the identified node receives the requestand creates the virtual disk object, by, for example, communicating withits corresponding the LSOM sub-module 350 to persistently store metadatadescribing the virtual disk object in its local storage. In step 425,the DOM sub-module 340, based on the virtual disk object blueprint,identifies those others nodes in cluster 110 that have been designatedto serve as the coordinator or owner for any component objects in thevirtual disk blueprint. The DOM sub-module 340 communicates (e.g., usingits RDT sub-module 345) with the DOM sub-modules 340 of the other nodesthat will serve as coordinators for the component objects and store thedata backing such component objects in their local storage. When suchDOM sub-modules 340 receive a request from the DOM sub-module 340 of thecoordinator of the virtual disk object to create their respectivecomponent objects, they, in turn in step 430, communicate with theirrespective LSOM modules 350 to allocate local storage for the componentobject (and its related metadata). Once such component objects have beencreated, their DOM sub-modules 340 advertise the creation of thecomponents to the in-memory metadata database of its CMMDS sub-module335 in step 435. In step 440, in turn, the DOM sub-module 340 for thecoordinator of the virtual disk object also advertises its creation toits CMMDS sub-module 335 to update the in-memory metadata database andultimately transmits an acknowledgement to the administrator (e.g., viathe master node communications back to virtual management platform 105).

FIG. 5 illustrates the handling of an I/O operation originating from aVM, according to one embodiment. When a VM running on a particular nodeperforms I/O operations to its virtual disk, the VM's guest operatingsystem, in step 500, transmits an I/O operation request intended for itsvirtual disk (through a device driver of the guest operating system)which, in step 505, is received by hypervisor 113 and ultimatelytransmitted and transformed through various layers of an I/O stack inhypervisor 113 to DOM sub-module 340 of VSAN module 114. In step 510,the I/O request received by DOM sub-module 340 includes a uniqueidentifier for an object representing the virtual disk that DOMsub-module 340 uses to identify the coordinator node of the virtual diskobject by accessing the in-memory metadata database of CMMDS sub-module335 (in certain embodiments, accessing the in-memory metadata databaseto look up a mapping of the identity of the coordinator node to theunique identifier occurs only when the virtual disk object is initiallyaccessed, with such mapping persisting for future I/O operations suchthat subsequent lookups are not needed). Upon identifying thecoordinator node for the virtual disk object, the DOM sub-module 340 ofthe node running the VM communicates (e.g., using its RDT sub-module345) with the DOM sub-module 340 of the coordinator node to request thatit perform the I/O operation in step 515. As previously discussed, incertain embodiments, if the node running the VM and the node serving ascoordinator of the virtual disk object are different, the two DOMsub-modules will communicate to update the role of the coordinator ofthe virtual disk object to be the node of the running VM. Upon thecoordinator's receipt of the I/O request, in step 520, its DOMsub-module identifies (e.g., by again referencing the in-memory metadatadatabase, in certain embodiments) those coordinator nodes for theparticular component objects (e.g., stripes) of the virtual disk objectthat are subject to the I/O operation. For example, if the I/O operationspans multiple stripes (e.g., multiple component objects) of a RAID 0configuration, DOM sub-module 340 may split the I/O operation andappropriately transmit corresponding I/O requests to the respectivecoordinator nodes for the relevant component objects that correspond tothe two stripes. In step 525, the DOM sub-module of the coordinator nodefor the virtual disk object requests that the DOM sub-modules for thecoordinator nodes of the identified component objects perform the I/Ooperation request and, in step 530, the DOM sub-modules of suchcoordinator nodes for the identified component objects interact withtheir corresponding LSOM sub-modules to perform the I/O operation in thelocal storage resource where the component object is stored.

In certain situations, it should be recognized that multiple clients(e.g., other VSAN modules 114 acting on behalf of running VMs) maysimultaneously send requests to perform I/O operations on a particularlocal storage resource located in a particular node at any given time.For example, the component objects (e.g., stripes, etc.) of differentvirtual disk objects corresponding to different VMs may be backed by thesame local storage on the same node. Upon receiving an I/O operation,the VSAN module 114 of such a node may place the I/O operation into astorage resource queue for processing. To reduce the possibility ofcongestion or overflow in the I/O queue for the local storage resourcecaused, for example, by multiple clients accessing component objects,the VSAN module 114 (via its LSOM sub-module, as previously discussed)monitors usage of the local storage resource and may issue a congestionmetric to the clients attempting to access the local storage. Thecongestion metric, discussed in greater detail below, provides a measureby which a client may calculate a delay prior to sending additional I/Orequests to the local storage resource.

FIG. 6 illustrates a method flow diagram for transmitting congestionmetrics to clients by a VSAN module, according to one embodiment. Forexample, components of different composite objects may reside in thesame node and contend for the shared local resources, such as SSD andmagnetic disk IOPS. As previously stated, the LSOM sub-module 350 ofeach node includes the configuration of the local component objects andpolicies as applied to each component. Further, LSOM sub-module 350monitors the resource usage of component objects to ensure that thecomponent objects adhere to the policy. If a client accessing acomponent object (e.g., other VSAN modules acting on behalf of runningVMs) exceeds the allotment of IOPS for that object, the VSAN module 114(through its LSOM sub-module 350) sends a congestion metric to theclient, upon which the client delays subsequent I/O requests. In step605, a VSAN module 114 of a node 111 in cluster 110 receives an I/Ooperation request originating from the client accessing a componentobject residing in node 111. For example, the VSAN module 114 (throughits DOM sub-module 340) may serve as the coordinator for a componentobject (e.g., stripe, etc.) of a virtual disk object corresponding tothe running VM. In step 610, once it receives the I/O operation request,the VSAN module 114 (through its LSOM sub-module 350) determines whetherthe IOPS capacity of the local storage resource backing the componentobject has been exceeded (e.g., resulting in contention and potentialcongestion) due to other clients that might be accessing the same localstorage to, for example, access other different component objects thatcomprise their corresponding virtual disk objects. If there is nocontention, then, in step 615, VSAN module 114 proceeds to handle theI/O operation request by communicating with its LSOM sub-module 350 toaccess the local storage resource.

However, if the IOPS capacity of the local storage resource has beenexceeded, then in step 620, the local LSOM sub-module 350 calculates acongestion metric for any client that is currently conducting I/O withthe local storage resource and has exceeded the IOPS reservationsspecified in their storage policies. In one embodiment, LSOM sub-module350 calculates the congestion metric using a time-weighted sum (e.g., bydecaying previous values and adding recent congestion measurements).Furthermore, in certain embodiments, LSOM sub-module 350 also maycalculate a different congestion metric for each client based on theclient's usage of the local storage resource relative to the usage byother clients of the local storage resource. In one embodiment, if theclient is accessing multiple local storage resources located indifferent nodes of cluster 110 (e.g., an I/O operation originating fromthe client is split into multiple I/O operations directed towardsdifferent component objects), VSAN module 114 may communicate with theVSAN modules 114 of the other nodes (through LSOM sub-modules 350) tocombine its congestion metrics with possible congestion metricsgenerated from the other VSAN modules residing in the other nodes toproduce an overall congestion metric for the client.

In step 625, the VSAN module 114 sends the congestion metric to theclients. Upon receipt, the clients, through DOM sub-module 340, use thecongestion metric to determine how long to delay subsequent I/Ooperation requests based on the amount of resources the client is usingand the amount of congestion described by the congestion metric. Thatis, rather than forcing a fixed period of delay on clients that areoverusing resources (such an approach may lead to unwanted oscillationsin usage), in certain embodiments, the congestion metric provides theclient with information to calculate a randomized delay period from adistribution proportional to the metric. Because the service costsrequired for read and write operations differ, the wait period requiredby the congestion metric may depend on the type and size of the I/Ooperation. For example, a congestion metric may require a longer delayperiod for a write operation than a read operation because generally,write operations are more computationally expensive than readoperations.

In one alternative embodiment, the VSAN module may issue a differentcongestion metric to every client accessing the local storage resourceregardless of whether such client is exceeding its reserved allocations.While the congestion metrics transmitted to clients exceeding theirreserved allocation may result in such clients delaying transmission ofsubsequent I/O operation requests, clients accessing the local storageresource within their reserved allocations may receive a congestionmetric (e.g., a zero metric) that permits them to send I/O operationrequests without delay. After the congestion has subsided, the VSANmodule 114 may stop transmitting congestion metrics after each clientrequest.

In an alternative embodiment, the host LSOM sub-module 350 calculates alocal measure of congestion and sends the congestion metric to eachclient without tailoring the congestion metric to the resource usage ofa particular client. Upon which clients delay requests if they areoversubscribing. It could then be up to the consuming client to respondto congestion if they're oversubscribing.

FIG. 7 illustrates a method for delaying an I/O operation request to theVSAN module during congestion, according to one embodiment. In step 705,a client (e.g., a VSAN module 114 of a node 111 supporting a VM) runningon a node of cluster 110 may receive a congestion metric as a result oftransmitting an I/O operation request to its virtual disk, as describedin FIG. 6. For example, the LSOM sub-module 350 of a client VSAN module114 may, in communicating with the LSOM sub-module 350 of the VSANmodule 114 of the node storing an object may receive such a congestionmetric. The DOM sub-module 340 of client VSAN module 114 may calculate atime delay based on the congestion metric. Upon receiving the congestionmetric, if the client VSAN module 114 (through its DOM sub-module 340)needs to transmit additional I/O operation requests, in step 710, itcalculates a time to delay prior to transmitting the request, forexample, based on a random distribution over a range, where the maximumvalue is a function of the congestion metric. In step 715, the clientVSAN module 114 waits for calculated delay prior to transmitting the I/Ooperation request. The wait period calculated from the congestion metricmay be based on the amount of IOPS the client is exceeding the reservedallocation. That is, clients exceeding the reserved allocation far morethan clients exceeding by a smaller amount may be required to delayrequests for a longer period.

Although one or more embodiments have been described in some detail forclarity of understanding, it will be apparent that certain changes andmodifications may be made within the scope of the claims. Accordingly,the described embodiments are to be considered as illustrative and notrestrictive, and the scope of the claims is not to be limited to detailsgiven herein, but may be modified within the scope and equivalents ofthe claims. For example, although a number of foregoing describedembodiments describe virtual machines as the clients that access thevirtual disks provided by the VSAN module, it should be recognized thatany clients, such as a cluster of non-virtualized host servers and/ornon-virtualized applications running therein may similarly utilize theVSAN module in alternative embodiment. Similarly, alternativeembodiments of the VSAN module may enable creation of high level storageobjects other than virtual disks, such as, without limitation, RESTobjects, files, file systems, blob (binary large objects) and otherobjects. Similarly, while the congestion prevention techniques describedin the foregoing embodiments related primarily to dealing withcongestion at local storage resources, alternative embodiments mayutilize similar techniques to reduce contention for memory, processingand/or networking resources that may arise, for example, if a singlenode operates as a coordinator for two different virtual disks currentlybeing accessed by two different VMs. In such embodiments, DOM sub-module340 may also monitor resource usage in CPU, memory, and networking todeal with contention for those resources in a manner similar to that ofLSOM sub-module 350 in monitoring resource usage such as IOPS andcapacity for the local storage resources (or alternatively, LSOMsub-module 350 may also be configured to monitor CPU, memory and/ornetworking usage). Similarly, while VSAN module 114 has been generallydepicted as embedded in hypervisor 113, alternative embodiments mayimplement VSAN module separate from hypervisor 113, for example as aspecial virtual machine or virtual appliance, a separate application orany other “pluggable” module or driver that can be inserted intocomputing platform in order to provide and manage a distributed objectstore.

As described, embodiments described herein measure congestion andcontrol quality of service to distributed resources. Advantageously, byissuing a congestion metric to clients while resources are contended andrequiring all clients that are overusing resources to delay requests,these embodiments provide predictability in accessing distributedresources and delivers low latency to clients that use resources asprovisioned. Further, delaying the requests at ingress to the resourceapplication rather than at the resource queues requires less complicatedqueuing logic than previous solutions have required.

The various embodiments described herein may employ variouscomputer-implemented operations involving data stored in computersystems. For example, these operations may require physical manipulationof physical quantities usually, though not necessarily, these quantitiesmay take the form of electrical or magnetic signals where they, orrepresentations of them, are capable of being stored, transferred,combined, compared, or otherwise manipulated. Further, suchmanipulations are often referred to in terms, such as producing,identifying, determining, or comparing. Any operations described hereinthat form part of one or more embodiments may be useful machineoperations. In addition, one or more embodiments also relate to a deviceor an apparatus for performing these operations. The apparatus may bespecially constructed for specific required purposes, or it may be ageneral purpose computer selectively activated or configured by acomputer program stored in the computer. In particular, various generalpurpose machines may be used with computer programs written inaccordance with the teachings herein, or it may be more convenient toconstruct a more specialized apparatus to perform the requiredoperations.

The various embodiments described herein may be practiced with othercomputer system configurations including hand-held devices,microprocessor systems, microprocessor-based or programmable consumerelectronics, minicomputers, mainframe computers, and the like.

One or more embodiments may be implemented as one or more computerprograms or as one or more computer program modules embodied in one ormore computer readable media. The term computer readable medium refersto any data storage device that can store data which can thereafter beinput to a computer system computer readable media may be based on anyexisting or subsequently developed technology for embodying computerprograms in a manner that enables them to be read by a computer.Examples of a computer readable medium include a hard drive, networkattached storage (NAS), read-only memory, random-access memory (e.g., aflash memory device), a CD (Compact Discs), CD-ROM, a CD-R, or a CD-RW,a DVD (Digital Versatile Disc), a magnetic tape, and other optical andnon-optical data storage devices. The computer readable medium can alsobe distributed over a network coupled computer system so that thecomputer readable code is stored and executed in a distributed fashion.

In addition, while described virtualization methods have generallyassumed that virtual machines present interfaces consistent with aparticular hardware system, the methods described may be used inconjunction with virtualizations that do not correspond directly to anyparticular hardware system. Virtualization systems in accordance withthe various embodiments, implemented as hosted embodiments, non-hostedembodiments, or as embodiments that tend to blur distinctions betweenthe two, are all envisioned. Furthermore, various virtualizationoperations may be wholly or partially implemented in hardware. Forexample, a hardware implementation may employ a look-up table formodification of storage access requests to secure non-disk data.

Many variations, modifications, additions, and improvements arepossible, regardless the degree of virtualization. The virtualizationsoftware can therefore include components of a host, console, or guestoperating system that performs virtualization functions. Pluralinstances may be provided for components, operations or structuresdescribed herein as a single instance. Finally, boundaries betweenvarious components, operations and data stores are somewhat arbitrary,and particular operations are illustrated in the context of specificillustrative configurations. Other allocations of functionality areenvisioned and may fall within the scope of one or more embodiments. Ingeneral, structures and functionality presented as separate componentsin exemplary configurations may be implemented as a combined structureor component. Similarly, structures and functionality presented as asingle component may be implemented as separate components. These andother variations, modifications, additions, and improvements may fallwithin the scope of the appended claims(s). In the claims, elementsand/or steps do not imply any particular order of operation, unlessexplicitly stated in the claims.

We claim:
 1. A computer-implemented method for handling a storageinput/output (I/O) operation originating from a virtual machine (VM) anddestined for a virtual disk, the VM running on virtualization softwareof a first host that is part of a cluster of host computers thatincludes the first host and a second host, each host having its ownlocal physical storage resources, the virtualization software of thefirst host containing a first driver, the method comprising: receiving,by the first driver, the I/O operation; retrieving from the I/Ooperation an identifier for a virtual disk object representing thevirtual disk, the virtual disk object being composed of virtualcomponent objects, the virtual component objects being mappable to anyavailable local physical storage resources throughout the cluster ofhost computers; determining, using the identifier, that a second host isa virtual disk coordinator host of the virtual disk object, by accessinga database of the first host, wherein the virtual disk coordinator hostcontrols access to the virtual disk object; performing one of: (1)transmitting, by the first driver, the I/O operation to a second driverof the second virtualization software of the second host, or (2)updating the database to set the first host to be the virtual diskcoordinator host of the virtual disk object; and executing the I/Ooperation by the virtual disk coordinator host by communicating withhosts that are the coordinators for the virtual component objects of thevirtual disk object.
 2. The method of claim 1, wherein the first hostimplements a hypervisor, and the hypervisor comprises the first driver.3. The method of claim 1, wherein the performing comprises transmitting,by the first driver, the I/O operation to the second driver of thevirtual disk coordinator host.
 4. The method of claim 1, wherein theperforming comprises updating the database to set the first host to bethe virtual disk coordinator host of the virtual disk object.
 5. Themethod of claim 1, wherein the executing of the I/O operation bycommunicating with hosts that are the coordinators for the virtualcomponent objects of the virtual disk object comprises: identifying, bythe virtual disk coordinator host, virtual component objects of thevirtual disk that are subject to the I/O operation, wherein a virtualcomponent object encompasses a data partition of the virtual disk;determining a component object coordinator host for each of the virtualcomponent objects, wherein the component object coordinator hostcontrols access to an associated virtual component object; transmitting,by the virtual disk coordinator host, the I/O operation to each of thecomponent object coordinator hosts; and performing, by each of thecomponent object coordinator hosts, the I/O operation in a local storageresource backing the virtual component object.
 6. The method of claim 5,wherein each host computer in the cluster contributes at least a portionof a corresponding local storage disk to form a pool of shared resourcesaccessible as the virtual disk.
 7. The method of claim 6, wherein atleast one section of the virtual disk is backed by a local storage diskhoused in the first host, and at least one other section of the virtualdisk is backed by a different local storage disk housed in a differenthost computer belonging to the cluster.
 8. The method of claim 7,wherein the virtual disk is organized as a RAID 0 stripe set, andwherein (1) the section of the virtual disk backed by the local storagedisk housed in the first host, and (2) the at least one other section ofthe virtual disk backed by the different local storage disk representdifferent stripes in the stripe set.
 9. A non-transitory computerreadable medium comprising instructions to be executed in a processor ofa computer system, and the instructions when executed in the processorcause the computer system to carry out a method of handling a storageinput/output (I/O) operation originating from a virtual machine (VM) anddestined for a virtual disk, the VM running on virtualization softwareof a first host that is part of a cluster of host computers thatincludes the first host and a second host, each host having its ownlocal physical storage resources, the virtualization software of thefirst host containing a first driver, said method comprising: receiving,by the first driver, the I/O operation; retrieving from the I/Ooperation an identifier for a virtual disk object representing thevirtual disk, the virtual disk object being composed of virtualcomponent objects, the virtual component objects being mappable to anyavailable local physical storage resources throughout the cluster ofhost computers; determining, using the identifier, that a second host isa virtual disk coordinator host of the virtual disk object by accessinga database of the first host, wherein the virtual disk coordinator hostcontrols access to the virtual disk object; performing one of: (1)transmitting, by the first driver, the I/O operation to a second driverof the second virtualization software of the second host virtual diskcoordinator host, or (2) updating the database to set the first host tobe the virtual disk coordinator host of the virtual disk object; andexecuting the I/O operation by the virtual disk coordinator host bycommunicating with hosts that are the coordinators for the virtualcomponent objects of the virtual disk object.
 10. The non-transitorycomputer readable medium of claim 9, wherein the first host implements ahypervisor, and the hypervisor comprises the first driver.
 11. Thenon-transitory computer readable medium of claim 9, wherein theperforming comprises transmitting, by the first driver, the I/Ooperation to the second driver of the virtual disk coordinator host. 12.The non-transitory computer readable medium of claim 9, wherein theperforming comprises updating the database to set the first host to bethe virtual disk coordinator host of the virtual disk object.
 13. Thenon-transitory computer readable medium of claim 9, wherein theexecuting of the I/O operation by communicating with hosts that are thecoordinators for the virtual component objects of the virtual diskobject comprises: identifying, by the virtual disk coordinator host,virtual component objects of the virtual disk that are subject to theI/O operation, wherein a virtual component object encompasses a datapartition of the virtual disk; determining a component objectcoordinator host for each of the virtual component objects, wherein thecomponent object coordinator host controls access to an associatedvirtual component object; transmitting, by the virtual disk coordinatorhost, the I/O operation to each of the component object coordinatorhosts; and performing, by each of the component object coordinatorhosts, the I/O operation in a local storage resource backing the virtualcomponent object.
 14. The non-transitory computer readable medium ofclaim 13, wherein each host computer in the cluster contributes at leasta portion of a corresponding local storage disk to form a pool of sharedresources accessible as the virtual disk.
 15. The non-transitorycomputer readable medium of claim 14, wherein at least one section ofthe virtual disk is backed by a local storage disk housed in the firsthost, and at least one other section of the virtual disk is backed by adifferent local storage disk housed in a different host computerbelonging to the cluster.
 16. The non-transitory computer readablemedium of claim 15, wherein the virtual disk is organized as a RAID 0stripe set, and wherein (1) the section of the virtual disk backed bythe local storage disk housed in the first host, and (2) the at leastone other section of the virtual disk backed by the different localstorage disk represent different stripes in the stripe set.
 17. Acomputer system comprising: a cluster of host computers that includes afirst host and a second host, each host having its own local physicalstorage resources, wherein the first host includes a virtual machine(VM) that runs on virtualization software of the first host, whereinvirtualization software of the first host includes a first driver of thevirtualization software of the first host; and a processor, wherein theprocessor is programmed to carry out a method of handling a storageinput/output (I/O) operation originating from the VM and destined forthe virtual disk, said method comprising: receiving, by the firstdriver, the I/O operation; retrieving from the I/O operation anidentifier for a virtual disk object representing the virtual disk, thevirtual disk object being composed of virtual component objects, thevirtual component objects being mappable to any available local physicalstorage resources throughout the cluster of host computers; determining,using the identifier, that a second host is a virtual disk coordinatorhost of the virtual disk object by accessing a database of the firsthost, wherein the virtual disk coordinator host controls access to thevirtual disk object; performing one of: (1) transmitting, by the firstdriver, the I/O operation to a second driver of the secondvirtualization software of the second host, or (2) updating the databaseto set the first host to be the virtual disk coordinator host of thevirtual disk object; and executing the I/O operation by the virtual diskcoordinator host by communicating with hosts that are the coordinatorsfor the virtual component objects of the virtual disk object.
 18. Thecomputer system of claim 17, wherein the performing comprisestransmitting, by the first driver, the I/O operation to the seconddriver of the virtual disk coordinator host.
 19. The computer system ofclaim 17, wherein the performing comprises updating the database to setthe first host to be the virtual disk coordinator host of the virtualdisk object.
 20. The computer system of claim 17, wherein the executingof the I/O operation by communicating with hosts that are thecoordinators for the virtual component objects of the virtual diskobject comprises: identifying, by the virtual disk coordinator host,virtual component objects of the virtual disk that are subject to theI/O operation, wherein a virtual component object encompasses a datapartition of the virtual disk; determining a component objectcoordinator host for each of the virtual component objects, wherein thecomponent object coordinator host controls access to an associatedvirtual component object; transmitting, by the virtual disk coordinatorhost, the I/O operation to each of the component object coordinatorhosts; and performing, by each of the component object coordinatorhosts, the I/O operation in a local storage resource backing the virtualcomponent object.